Abstract Exit from host cells is an essential step in the life cycle of intracellular pathogens including Toxoplasma gondii, the agent of human toxoplasmosis. Related malaria parasites also egress from host cells after replication, and they additionally traverse cells by escaping from a transient vacuole in a manner similar to egress. Recent work has identified critical roles for pore-forming proteins and phospholipases in cell egress and traversal. But how these proteins facilitate egress and the extent to which they collaborate to disrupt membranes is unknown. Also, although recent reports suggest that acidification of the parasite-containing vacuole augments activity of the pore-forming protein(s), the proton pumps mediating acidification have not been identified. Precisely how acidification facilitates pore-formation and egress is also unclear. The absence of such knowledge precludes rationale design of interference strategies to alter the course of infection and disease. We will address these key gaps by identifying the proton pump(s) responsible for vacuole acidification (Aim1), defining the structural basis for pH- augmented membrane binding by the pore-forming protein Perforin-Like Protein 1 (Aim 2), and measuring pH-dependent cooperativity between Perforin-Like Protein 1 and an LCAT phospholipase for membrane disruption during egress. The proposed studies will advance the field by delivering insight that is broadly relevant to egress by apicomplexan parasites and other cytolytic pathogens.